National Highways say the M1 will be closed southbound between Junction 42 (Lofthouse) and 41 (Wakefield / Morley) "for several hours" because of a serious crash.
The incident happened between two vehicles at around 4.40pm this afternoon.
It's being reported there are serious injuries. The road is closed southbound while West Yorkshire Police carry out an investigation.
There are long delays approaching the closure with both the M1 southbound and the M621 clockwise carriageway having become congested as a result.
Drivers are being urged to use the A1 instead if they are going south and coming from further afield.